This invention relates to a hydraulic control system for use in vehicles having an engine which not only powers the vehicle but also drives a pump to provide hydraulic fluid under pressure for the hydraulic control system and more particularly to such a system including an electrical circuit and an electrically operated motor pump unit for automatically supplying hydraulic fluid under pressure to the control system if the engine or main pump fail in operation.
For example, the weight of various self-propelled vehicles of the type utilized in the earth moving industry make it necessary to provide power steering to aid the vehicle operator in controlling the vehicle. Such power steering conventionally takes the form of a hydraulic circuit including steering control means actuated by hydraulic fluid under pressure and the usual practice is to provide a pump driven by the vehicle engine to provide the pressurized hydraulic fluid. Thus, if the engine of the vehicle fails in operation, it becomes extremely difficult to control the vehicle while it is being brought to a stop and if the engine cannot be started, then it will be difficult to control the vehicle while it is being moved to a place where repairs can be made.
It has been proposed in the prior art to provide an auxiliary or emergency hydraulic system including an electrically driven motor pump unit to provide hydraulic fluid under pressure to the control system if the engine or main pump is inoperative for any reason. For example, U.S. Pat. No. 3,153,462 discloses such an electrically powered auxiliary or emergency system which is manually actuated by the operator upon the occurrence of an emergency condition. However, by the time the operator realizes that an emergency condition has occurred, it may be too late to energize the emergency system in time to avoid an accident.
Thus, it is an object of this invention to provide an electrical control circuit for automatically actuating the electric motor of an auxiliary or emergency system for supplying hydraulic fluid under pressure if the engine or the main hydraulic pump should fail for any reason.
Automatic control circuits of this type have been proposed as disclosed in U.S. Pat. Nos. 2,954,671 and 3,434,282. However, such previously proposed control circuits are undesirable in that the auxiliary or emergency system is actuated whenever the engine of the vehicle is started.
It is an object of this invention to provide a control system for automatically actuating the emergency hydraulic system only upon a failure of the engine or main hydraulic pump after the engine has been in normal operation.
In addition, the automatic operation of control systems proposed in the prior art has depended upon means for sensing the pressure of the hydraulic fluid provided by the main hydraulic pump whereas the fluid flow from the main hydraulic pump is a more important criteria with respect to the satisfactory operation of the main hydraulic system. This is particularly true in hydraulic systems wherein a constant flow of hydraulic fluid is maintained into and out of the reservoir and the pressures encountered in the system vary depending on the state of actuation of the system.
Thus, it is a further object of this invention to provide a control system sensitive to the flow of hydraulic fluid from the main hydraulic pump for automatically actuating the emergency hydraulic system when such flow falls below a given level.